Ancient Egyptians Tracked Eclipsing Binary Star Algol

Turn your telescope to the constellation of Perseus and you might note an unusual star called Algol, dubbed the “Demon Star” or the “Raging One.” You wouldn’t notice anything much different at first, unless you happened to be looking during a window of a few hours — every 2.867 days — when Algol’s brightness visibly dims.

This unusual feature was first noticed back in 1667 by an astronomer named Geminiano Montanari, and later confirmed — with a proposed possible mechanism — in 1783 by John Goodricke, who precisely measured the period of variability: it dims every 2.867 days.

This isn’t the first time people have hypothesized that Algol’s variable nature was known prior to its discovery in the 17th century. Certainly it was a familiar object, prominent in mythology and lore. In the second century, Ptolemy referred to Algol as the “Gorgon of Perseus,” and associated it with death by decapitation. (In Greek mythology, the hero Perseus slays the snake-headed Gorgon, Medusa, by chopping off her head.)

Other cultures also associated the star with violence and bad fortune. It’s no coincidence that H.P. Lovecraft marked the onset of his final battle in the 1919 short story, “Beyond the Wall of Sleep,” with the appearance of a nova near Algol.

But the Helsinki researchers go beyond mythology and conjecture and provide a solid statistical analysis, based on historical documentation.

Goodricke proposed that Algol’s periodic variability was due to an eclipsing factor: namely, an orbiting dark body occasionally passed in front of the star, dimming its brightness temporarily.

Alternatively, he suggested that Algol itself had a darker side that turned toward the Earth every 2.687 days.

His hypothesis wouldn’t be confirmed until 1881, when Edward Charles Pickering discovered that Algol is actually a binary star system: there were two stars circling together, Algol A and Algol B.

Even more intriguing: it was an “eclipsing binary,” i.e., one in which the dimmer star in the system occasionally passes in front of its brighter sibling, dimming the latter according to predictable periods. Goodricke’s hypothesis was correct.

Actually, astronomers now know that Algol is a triple-star system, with a third star, Algol C, located a bit further out from the main pair, with a larger orbit.

All of this is necessary background for understanding the conclusions of the Helsinki scientists. The whole point of tracking the heavens so meticulously, for the Egyptians, was to make predictions about the future, dividing the calendar into “lucky” and “unlucky” days. The Cairo Calendar, while badly damaged, nonetheless contains a complete list of such days over a full year, circa 1200 B.C.

How did the Egyptians decide how to rate specific days? That’s a mystery. But the Finnish team took the raw data and reassembled it into a tie series, then used statistical techniques to determine the cycles within it. There were two significant periodic cycles. One was 29.6 days, very close to current estimates of a lunar month (29.53059 days).

The second periodic cycle was 2.85 days. Lead author Lauri Jetsu and her colleagues argue that this corresponds to Argol’s variable period. It’s suspiciously close to the 2.867 period Goodricke measured back in 1783.

Close, yes, but it’s not a precise match, which is problematic. The Egyptians weren’t known to be sloppy in their astronomical calculations. They should have been able to pinpoint a value much closer to Goodricke’s — unless, say, Algol’s period changes over time.

There is some evidence that this might be the case, possibly due to the presence of the third star in the Algol system. Calculating the behavior of a two-body system is one thing; grappling with the dreaded “three-body problem” is quite another, particularly since astronomers are only working with roughly 300 years of data. Algol looks like it’s living up to its “Demon Star” moniker.

That’s where Jetsu et al‘s paper might prove to be more than just an intriguing historical oddity. It provides some missing data from 3000 years ago, which could help astronomers further constrain their models for Algol’s variable behavior.